TIME has learned that sometime today the FBI plans to send a classified Intelligence Bulletin to 18,000 state and local law enforcement agencies over its secure telecommunications network, advising officials to pay attention to suspicious activities around nuclear power plants, and including people who are spotted photographing them and light aircraft flying near them. There have been a number of past instances in which light aircraft have flown too close to nuclear plants, and while the aircraft have turned out to be pilots who have flown off course or simply been curious about nuclear plants, all such sightings must be reported to the FAA by tail number immediately.

FBI officials say they have no specific intelligence that these plants are imminent targets of a terrorist attack. Still, says one FBI official, "It's one of the vulnerabilities, and it's something that's taken very seriously."